Past designs for solar/electric conversion systems have typically employed pressed fresnel lenses with long focal lengths and small acceptance angles. These systems have sufficiently high costs as not to be suitable for high volume manufacture.
Pressed fresnel lenses are used because sharp edges and good surface finish is possible, but this is at the expense of using straight sided or non-undercut lens tips or facets. Long focal lengths are generally necessary with these straight vertical sided lenses in order to keep efficiency high. Some systems have used straight sided pressed lenses and then curved the lenses after manufacture in a secondary operation, in order to reduce focal length without sacrificing efficiency. Both of these approaches are costly. Also this construction employs side/lens joints which are wasted from a light collection point of view and require increased cost in surface areas.
High efficiency photovoltaic cells are normally made with grid lines on their surface. These grid lines block a substantial amount of light from reaching the photovoltaic cells therebeneath. Thus, wherever previous cells have been oriented at approximately 90 degrees to the main axis of the lens and since the grid lines thereon serve to reflect light back outside the lens to be lost, the efficiency of the system is significantly reduced.
Solar concentrating systems which must track the sun typically have a plurality of modules which are spaced substantially apart, for example, in some instances by approximately 100%, in order to avoid shadowing of one lens module from another when the sun is not directly overhead, and hence, in this type of system the space left between modules has been wasted.
The present invention discloses a photovoltaic generating system organized and arranged to be made and operated at far lower cost and substantially increased efficiency with reduced losses and increased acceptance angles. A short focal length fresnel lens and sides are extruded in one (or more) pieces and in such a way that the fourth side is made up of a heat sink which carries the photovoltaic cell thereon. The f resnel lens is shaped with a curved top surface and lower lens facets with undercut lens tips to reduce short focal length losses caused by shadowing between facets. These facets are proportioned to minimize radial losses and material. (i.e., average facet radial depth to focal length is preferably in the range of 0.8% to 3.5%).
Optical efficiency is further enhanced by utilizing the extruded side walls of the module as light pipes to carry light from the side/lens joint area via the side walls to a cell.
The extruded lens is designed with a relatively low top curvature in order to reduce material and reflective losses while the lower surface of the lens is more sharply curved than the top surface. This causes the direction of light entering to be substantially different or displaced from the angle of light leaving the lens. As disclosed herein the refraction averaged across the face of the lens runs on the order of approximately 15 degrees.
The outside edges of the fresnel lens or top edges of the side walls which are normally joints and hence areas of losses, are shaped to collect this otherwise lost light and channel it into the light pipe sides. The light passing via the side walls is directed to the photovoltaic cell by a tapered end section which is reflective on the outside surface so that the internally refracted light eventually exits the tapered section adjacent the cell.
The acceptance angle of the system is greatly increased by making the sides of the module inwardly reflective so that light entering via the fresnel lens which strikes the sides of the module and would otherwise be lost, is reflected by the sides back to the photovoltaic cell and utilized.
A secondary concentrating lens can be employed to increase the concentration ratio, giving more uniform flux distribution either as a line or a point focus, lowering the amount of cell area required and decreasing the overall cost of the entire system.
The efficiency of the system can be increased by replacing the usual single cell with two cells, each constructed with a number of spaced parallel conductors with tops of reflective material. These two cells are disposed respectively in intersecting planes. In this way, lost reflected light either from the cell or its overlying conductors is directed to a second cell and a majority of this otherwise reflected or lost light is used.
A further increase in efficiency is achieved by using two different types of cells, wherein each "type" of cell absorbs or responds to a different particular range of wavelengths of light. Thus, with one cell of silicon and the other of a material such as aluminum gallium arsenide (AlGaAs), light not used by one of the two cells is reflected to the second cell to be used by it and a higher percentage of the light spectrum is converted to electrical energy.
While applicant has referred throughout to conversion of solar energy to electrical energy it is to be understood that conversion from solar energy to energy in another form such as steam can be achieved by the same system by replacing the photovoltaic cell with a coil of tubing material supplied by a body of water.
The V cells can be made either to reflect selective wavelengths from their surface or to absorb light and reflect unused wavelengths from the reflective back surface of the cell. A still further increase in efficiency and cost effectiveness in concentrating systems has been achieved by utilizing the otherwise wasted space located between modules which space is normally present to prevent one module from shadowing the other. To reduce this loss the intermediate space between adjacent pairs of concentrators is filled with flat plate type photovoltaic cells arranged in either a V cell configuration or in a flat configuration with a clear thin cover shaped to include a plurality of v-shaped surfaces. The alternately spaced flat plates absorb the otherwise lost light not used by the concentrators, thereby increasing the efficiency of the overall system.
Thus, a general object of this invention is to provide a solar concentrator and system having substantially enhanced efficiency at lower cost.
Another object of the invention is to increase the efficiency of the flat plate collectors by either a V shaped configuration of the cells themselves or to have the cells arranged to be flat but to have a glass or plastic cover shaped in a series of V's so that surface reflections are reduced and the efficiency of the system is thereby increased.
It is yet another object of the invention to provide a linear concentrating collector of low concentration in which light is simultaneously directed to the top and underside of the cell. A substantial increase in concentration is then possible without reducing acceptance angles. This collector can have acceptance angles of plus or minus 25 degrees by plus or minus 90 degrees and hence will not need to track the sun, becoming far less expensive and more reliable.
An additional object of the invention is to provide a heat sink formed to support the cell from beneath while leaving a large portion of the underside of the cell exposed to sunlight.
A further object of the invention is to provide a short focal length fresnel lens.
Another object of the invention is to provide a unitary extruded body comprising a fresnel lens and a pair of side walls depending therefrom.
Yet a further object of the invention is to provide a totally internally reflecting secondary lens.
Yet an additional object of the invention is to provide a module with internally reflecting sides thereby forming a wide acceptance angle to the unit.
A further object of the invention is to provide a solar concentrator which operates with single axis tracking of the sun and provides a relatively wide tolerance at low cost.
Yet a further and additional object of the invention is to provide a staked or snap-on heat sink secured to said body to form a closure of the module between the spaced edge margins of the side walls.
An additional object of the invention is to make a large system to operate relatively inexpensively even with large tracking errors.
Yet a further object of the invention is to provide light conductive side walls to a unit of the kind described.
An additional object of the invention is to make a fresnel lens with undercut facets, in order to obtain a short focal length.
Yet another object of the invention is to provide a system and unit with selective wavelength reflection capability so as to absorb more of the light which strikes one or the other of the two cells of a V-cell element.